I know that there is a Battletech one, but Ive heard it's overly complex. I've also heard of old Macross and Robotech ones, but I don't know much about them, and I'm always Dubious about older games. And finally, I know about Robotech RPG Tactics, but it's not an RPG despite having "RPG" in the title.

Robotech RPG Tactics is a wargame derived from the Robotech RPG - the 'RPG' is in the title to pay homage to its origins.

"Heavy Gear" comes to mind. There's also some Mecha elements in Cthulhutech, if you like mixing Mecha with horror and magic. "MechWarrior" has an RPG as well, though I'm not sure how up to date it is.

There are a few Mecha-based RPGs using the FATE system - "Mecha vs Kaiju" (title says it all), "Apotheosis Drive X" (a Gundam-esque setting) and my personal favourite, "Iron Edda: War of Metal and Bone" which uses Dwarven-crafted Mecha in a Norse Mythology setting.

This is the tip of the iceberg, just literally ones I could pull off the top of my head. There's more out there... lots more.

Mechwarrior, Heavy Gear, and Mekton II are all decent and will scratch a certain itch. Of them, I think Heavy Gear is probably the most interesting in terms of background and mechanics, although the original setting for Battletech/Mechwarrior (AD 3025, pre-Clans) is pretty cool too.

I have always lurked around here, love seeing all this awesome stuff, and rarely visit the RPG section of MWG...but you know, I have the same issue-there isn't a lot of Mecha-focused RPG's, especially with some focus into Wargaming even involved with it. Heavy Gear and Battletech really were the only ones.